1. Field of the Invention
This invention relates to leadframes and methods of fabrication of such leadframes.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices, semiconductor integrated circuit chips are generally secured to leadframes with bond pads on the chips being coupled to inner lead fingers of the leadframe by way of gold wires. The leadframes make the electrical connection of the integrated circuit to the circuit board possible. To accomplish this purpose, the leadframes provide gold wire bondable inner lead surfaces to allow connection of the gold wire from the integrated circuit bond pad to the leadframe inner lead by bonding, as well as a flat surface to which the integrated circuit chip is attached. The chip inner leads with wire bonds are then encapsulated in plastic and the external leads, which remain external to the plastic encapsulation, must then be solderable to the printed circuit board surface, typically using a solder paste.
In the prior art, the external leads, after encapsulation of the chip and internal leads with gold wires, have been either dipped in molten solder, which is generally an alloy of tin and lead, or plated with the tin/lead solder to preserve the solderability of the external leads. The internal lead tips has been silver spot plated during leadframe manufacture prior to chip attach. A lead finish was thereafter introduced into the art based upon nickel and palladium which eliminated the use of solder and consequently the lead and tin contained in the solder from the leadframe. It also eliminates the silver spot plating of the internal leads. All plating is provided by the leadframe manufacturer, so there is no need for plating at the assembly site. This has proven over time to be a stable and robust lead finish with demonstrable benefits. The drawback to this system is that the nickel layer is generally thick, being on the order of 40 to 120 microinches, to prevent copper diffusion from the base metal to the surface, nickel is a target, albeit of lower importance than lead, of environmental concern. The deposition of nickel is also time consuming and requires rather extensive and expensive waste treatment facilities. It is therefore apparent that elimination or reduction of the nickel and/or replacement of the nickel with a more environmentally and/or economically friendly material is highly desirable.